The present invention relates to systems and methods for measuring the thickness of sheet-like bio-materials and, in particular, to an improved pericardial tissue mapping and marking system and methods therefore, especially for measuring tissue to be used for making prosthetic heart valve leaflets.
Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid, and pulmonary valves. Prosthetic heart valves can be used to replace any of these natural valves. The two primary types of prosthetic heart valves known in the -art are mechanical valves and bio-prosthetic valves. Bio-prosthetic valves may be formed from an intact, multi-leaflet porcine (pig) heart valve, or by shaping a plurality of individual leaflets out of bovine pericardial tissue or other materials, and combining the leaflets to form the valve. The present invention provides systems and methods for assessing and preparing material for leaflets in bio-prosthetic valves.
The pericardium is a sac around the heart of vertebrate animals, and bovine (cow) pericardium is commonly used to make individual leaflets for prosthetic heart valves. The bovine pericardium is first harvested from the animal and then chemically fixed to crosslink collagen and elastin molecules in the tissue and increase the tissue durability, before being cut into leaflets. Various physical characteristics of the tissue may be examined before or after fixation.
One drawback faced by a patient having an implanted bio-prosthetic heart valve is the potential for calcification of the leaflets if the valve remains in place for an extended period of time (more than ten years). Calcification tends to make the leaflets less flexible. A significant amount of research has been accomplished in mitigating calcification of bovine pericardial leaflets to lengthen the useable life of the heart valve. Calcification may reduce the performance of the heart valve, and thus, the highest quality materials and design in the heart valve is required to forestall a failure of the valve from excessive calcium deposits.
One aspect of designing heart valves which is very important in improving their performance is the selection of the pericardial tissue used in the leaflets. In all heart valves, the natural action of the flexible heart valve leaflets, which seal against each other, or co-apt, is desirable. The difficulty in simulating the leaflet movement of an actual heart valve (especially a mitral valve) in a prosthetic valve is that the leaflets used are xe2x80x9cinanimate.xe2x80x9d There are no muscular attachments to the leaflets as in the natural valve, and the prosthetic leaflets must co-apt to function properly solely in response to the fluid pressures within the heart chambers. Indeed, natural coaptation of the leaflets in bio-prosthetic valves comprising a plurality of individual leaflets sewn together is particularly difficult, even when compared to inanimate but intact valves, such as harvested porcine valves.
Despite the drawbacks of artificial heart valve material, over twenty years of clinical experience surrounding implanted artificial heart valves has produced a proven track record of success. Research in extending the useful life of the bio-prosthetic valves continues, however. Much of this research involves the mechanical properties of fresh or fixed bovine pericardium.
A good discussion of the various physical properties of fixed bovine pericardium is given in Simionescu, et al, Mapping of Glutaraldehyde-Treated Bovine Pericardium and Tissue Selection For Bio-prosthetic Heart Valves, Journal of Bio-Medical Materials Research, Vol. 27, 697-704, John Wiley and Sons, Inc., 1993. Simionescu, et al., recognized the sometimes striking variations in physical properties of the pericardial tissue, even in the same pericardial sac. Their research mapped out areas in individual pericardial sacs and tested those areas for various properties to determine the optimum area on the tissue from which to cut heart valve leaflets. Simionescu, et al. measured the thickness of the pericardial sacs at 5 mm increments and plotted the resulting values on a paper template identical in shape and size to the sac. On other templates, parameters such as the suture holding power, fiber orientation, and shrinkage temperature were mapped. After superimposing all of the templates, optimum areas from which to cut leaflets were identified. Simionescu, et. al., utilized a manual thickness measuring tool similar to that described below with respect to FIG. 1.
A number of steps in a typical commercial process for preparing pericardial tissue for heart valve leaflets is illustrated in FIG. 1. First, a fresh pericardial sac 20 is obtained from a regulation slaughterhouse. The sac 20 is then cut open along predetermined anatomical landmarks, as indicated at 22. The sac is then flattened at 24 and typically cleaned of excess fat and other impurities. After trimming obviously unusable areas, a window 26 of tissue is fixed, typically by immersing in an aldehyde to cross-link the tissue, and then quarantined for a period of about two weeks. Rough edges of the tissue window 26 are removed and the tissue bio-sorted to result in a tissue section 28. The process of bio-sorting involves visually inspecting the window 26 for unusable areas, and trimming the section 28 therefrom. Subsequently, the section 28 is further cleaned as indicated at 30.
The section 28 is then placed flat on a platform 32 for thickness measurement using a contact indicator 34. The thickness is measured by moving the section 28 randomly around the platform 32 while a spindle 36 of the indicator 34 moves up-and-down at various points. The thickness at each point is displayed at 38 and recorded mentally by the operator. After sorting the measured sections 28 by thickness, as indicated at 40, leaflets 42 are die cut from the sections, with thinner leaflets 42 generally being used for smaller valves, and thicker leaflets being used for larger valves. Of course, this process is relatively time-consuming and the quality of the final leaflets is dependent at several steps on the skill of the technician. Moreover, the number of leaflets obtained from each sac is inconsistent, and subject to some inefficiency from the manual selection process.
More recently, Baxter International Inc. has added a sophisticated leaflet selection method into its tissue valve manufacturing process. The method includes applying a load to each leaflet, as opposed to pericardial tissue in bulk, and recording the strain response. The results of the load test in combination with a droop test can be used to group similar leaflets. Such a method is disclosed in U.S. Pat. No. 5,961,549 to Huynh, issued Oct. 5, 1999, and entitled, xe2x80x9cPROSTHETIC HEART VALVE LEAFLET SELECTION METHOD AND APPARATUSxe2x80x9d. Although this method improves the quality of the resulting combination of leaflets, because of the existing inefficiencies in the process of supplying tissue from which to cut the leaflets, the subsequent filter of leaflet selection further reduces the total usable leaflet output such that costs are increased.
Despite much research into the characteristics of bovine pericardium and leaflets, there remains a need for a system and method for rapidly and reliably characterizing material, especially pericardial tissue, for use in fabricating heart valve leaflets.
The present invention provides a method of measuring the thickness of a bio-material sheet for use in bioprostheses, such as heart valves, grafts, and the like. The method involves mapping the thickness of the sheet and marking the sheet into areas or zones of similar thickness. The measuring, mapping, and marking steps can all be carried out automatically with a system that receives the sheet and translates it under a measurement head and a marking head, with the mapping function being performed by a connected computer and associated software. In a preferred embodiment, the bio-material sheet is bovine pericardium and from which heart valve leaflets are to be cut. The method further may include providing input as to a preferred thickness needed, and selecting the zones based on that input to maximize the preferred thickness marked.
In one aspect of the invention, a method of measuring the thickness of a bio-material sheet comprises first flattening the sheet on a sanitary surface, simultaneously measuring the thickness of a plurality of points on the flattened sheet, and automatically recording the measured thicknesses of the plurality of points. The step of simultaneously measuring desirably includes measuring at least three points, and more preferably at least ten points, on the flattened sheet. Further, the step of simultaneously measuring may occur more than once, wherein the plurality of points in each step of simultaneously measuring is arrayed along a line, and wherein each line is spaced from the line in a preceding or subsequent step of measuring so as to obtain a two-dimensional array of measured points on the sheet.
In another aspect of the invention, the method may further include providing a measurement head positioned normal to the surface, and relatively displacing the surface and measurement head in a direction parallel to the surface between each successive step of simultaneously measuring. A base may be provided upon which both the surface and measurement head are mounted, and the step of relatively displacing may comprise translating the measurement head relative to the base between each successive step of a simultaneously measuring. Desirably, a programmable controller controls movement of the measurement head.
The step of simultaneously measuring may include simultaneously contacting a plurality of points on a surface of the sheet facing away from the surface, preferably with a plurality of coil-driven shafts and monitoring the position of each shaft. Or, the step of simultaneously contacting includes simultaneously contacting the surface of the sheet with a plurality of free-sliding pins and monitoring the position of each pin.
In another aspect, the present invention provides a method of mapping the topography of a bio-material sheet by first providing a measuring system including a sanitary surface and a measurement head positioned normal to and spaced from the surface, wherein the measurement head includes a plurality of sensors adapted to measure distance along spaced axes normal to the surface. The sheet of bio-material is flattened on the surface, and the thickness of the sheet at a plurality of points is measured using the sensors. The thickness data is then used to create a topographical map of the sheet. The method, further may include marking the sheet to indicate the thickness of the plurality of points corresponding to the topographical map. Also, areas of different thickness may be marked on the sheet. In a preferred embodiment, the sheet is bovine pericardium and the step of marking areas of different thicknesses includes identifying discrete zones of similar thickness that are large enough from which to cut a heart valve leaflet. The method may involve controlling the marking with a computer, supplying the computer with information regarding a preferred thickness of heart valve leaflet, and controlling the marking based on the preferred leaflet thickness information so as to maximize the number of discrete zones of the preferred leaflet thickness that are marked.
In a still further aspect, the invention provides a method of automated mapping of a bio-material sheet to indicate discrete zones from which to cut heart valve leaflets, comprising measuring the thickness of a plurality of points on a flattened sheet, automatically recording the measured thicknesses of the plurality of points, and using the recorded thicknesses to mark discrete zones of the sheet that are large enough from which to cut heart valve leaflets. The method desirably includes determining an acceptable thickness range for each of a number of sizes of heart valve leaflets; and determining an acceptable minimum size of the discrete zones for each of a number of sizes of heart valve leaflets. Where the plurality of points is a two-dimensional array, a plurality of planar units are each centered on one of the measured points, and each discrete zone comprises a plurality of contiguous planar units. Each discrete zone may be selected so that at least some of the planar units within that discrete zone have a measured thickness within the acceptable thickness range for the corresponding heart valve leaflet. Finally, the method further may include marking the discrete zones on the sheet so as to maximize the number of discrete zones of the preferred leaflet thickness that are marked.
A system for measuring the thickness of a bio-material sheet is also provided, comprising a base adapted to be fixed with respect to a support floor, a sanitary platen mounted on the base, and a measurement head mounted on the base and positioned normal to and spaced from the platen. The measurement head includes a plurality of sensors adapted to measure distances along spaced measurement axes disposed normal to the platen, and the sensors are adapted to measure the thickness of a bio-material sheet that has been placed on the platen. The system may further include a movable carriage on which is defined the platen, and a first mechanism configured to relatively displace the platen and measurement head across the platen to enable each sensor to measure the thickness of the sheet at more than one point. Desirably, the platen defines a planar surface on which the bio-material sheet is measured, and the first mechanism enables relative linear translation of the planar surface and measurement head, preferably relative to the base along a first axis parallel to the planar surface. A second mechanism may be provided to relatively displace the planar surface and measurement head along a second axis parallel to the planar surface and perpendicular to the first axis, and desirably the second mechanism translates the planar surface relative to the base along the second axis. A third mechanism may permit relative displacement of each of the sensors on the measurement head along the respective parallel measurement axes disposed normal to the planar surface.
In the system as described above, the sensors each preferably include a tip for contacting a surface of the sheet facing away from the platen. Further, the third mechanism desirably includes a plurality of coil-driven shafts, one per sensor, with the tips positioned at the end of the shafts, and a position detector for monitoring the position of each shaft.
Still another aspect of the invention is a system for topographically mapping the thickness of a bio-material sheet, comprising:
a measurement head adapted to measure the thickness of a plurality of points on the sheet;
a computer connected to receive data corresponding to the thickness of the sheet at the plurality of points; and
software loaded on the computer and configured to analyze the data and identify discrete areas of similar thickness on the sheet.
The system may also include a marking head for marking the discrete areas of similar thickness directly on the bio-material sheet. Where the bio-material sheet is suitable for forming heart valve leaflets therefrom, the system further includes a human-machine interface enabling the computer to be supplied with a value of a preferred thickness of heart valve leaflet. The software is configured to control the marking head to maximize the number of discrete zones of the preferred leaflet thickness that are marked. Preferably, the human-machine interface comprises a touch-screen monitor, and the marking head comprises an ink jet type of dye dispenser.
In a particularly preferred embodiment, therefore, the present invention provides a three-axis computer-controlled positioning system, an array of programmable linear actuators, a high-performance dispenser for tissue marking, a PC-based data acquisition and processing system, a human-machine interface (HMI), and a central programmable logic controller (PLC) to control the overall system. A thickness measurement is made by placing the tissue sample on a flat stainless-steel measurement plate. Mechanical holders may or may not be used to retain the tissue sample on the plate. The thickness of the tissue sample is determined by touching the tissue with the actuator rod in a raster pattern across the surface of the sample. A three-axis motion system is used to translate the linear actuators in one direction (X) while the position of the measurement plate (and thus the sample) is incremented along a second axis (Y). The actuators and the dispensing head translate along the third axis (Z) with respect to the plate for tissue measurement and/or marking. At each point in the measurement, the positions of the actuator rods are digitized and stored. Following data collection, this information is processed to calculate the thickness of the tissue at each point in the measurement process. Based on these measurements, a thickness map is generated and used to identify tissue thickness areas for tissue zone marking and cutting.
A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.